1. Field of the Invention
The present invention relates to vector control of an induction machine with a selectable number of poles. The invention also relates to vector control of a multi-phase induction machine starter alternator having a selectable number of poles.
2. Discussion of the Related Art
Toroidally wound induction machines have been used for various applications. one of the methods for changing the number of machine poles without using any contactors or mechanical switches is called Pole-Phase Modulation (PPM).
The method of using PPM is only briefly discussed below. Basically, PPM is a method of changing the number of pole pairs of an AC machine winding without the need for contactors or mechanical switches. By its concept, mathematically, PPM is a generalized form of the Pole-Amplitude Modulation (PAM) method:                The number of phases with PAM is fixed, while with PPM it can vary.        PAM allows for pole change only in the ratio p:(p-1) while PPM provides for an arbitrary ratio.        
The PPM implementation consists of selecting the number of pole pairs by controlling the phase shift between currents in the elementary phases, where each elementary phase consists of a coil or a group of coils connected in series. Since all conductors of the winding are energized at each pole pair combination, a machine with PPM has much better utilization of active material than a regular machine with separate windings for each pole pair.
As opposed to Dahlander's connection, which allows only one, 2:1, ratio between the number of pole pairs created by a single winding, the number of pole pairs in PPM is arbitrary. A PPM winding is a generalized Dahlander winding with an arbitrary number of two or more different pole pairs. The Dahlander winding is usually built with full pitch at lower speeds of rotation, and, therefore, with half the pole pitch, i.e. y+τp/2 at higher speeds of rotation (y denotes here the winding pitch and τp is the pole pitch, both expressed in the number of slots). The PPM winding, on the other hand, is always built to have full pitch at higher speeds, when the number of pole pairs at lower speeds is odd, and a shortened pitch at higher speeds of rotation, when the number of pole pairs at lower speeds is even.
The number of pole pairs p is a function of the total number of stator slots N, the phase belt q, and the number of phases m according to the equation:p=N/2qm  (1)Where p and m must be obviously integers, and q is usually an integer. This means that an m-phase machine with N slots can be built having several pole pairs, the numbers of which depend on the value of q. Basically, the PPM method uses the inverter switches to re-connect machine coils in the desired pole-phase configuration. The principles of PPM will be illustrated using an example of two different numbers of pole pairs generated by a single winding. Since the winding configuration in PPM varies as a function of the number of pole pairs at lower speeds, the principles of PPM will be illustrated on a 72 slots, 4/12 pole toroidal machine. However, this example in no way restricts the generality of the PPM method.
By way of example, a toroidally wound induction machine having 72 slots on the stator is discussed. Each elementary coil uses one slot, and two adjacent elementary coils connected in series form a coil, so that the total number of coils is 36.
With the 12-pole connection, the machine has three coils per pole; the coils are configured in three phases, denoted A, B and C, so that each phase has 12 coils. The coil placement and direction of winding are shown in FIG. 1. The (+) or (−) sign associated with each phase, specifies whether the coil is wound in a positive or negative direction. For example, phase A consists of coils 1, 7, 13, 19, 25 and 31 wound in a positive direction and coils 4, 10, 16, 22, 28 and 34 wound in a negative direction and all connected in parallel. FIG. 2 shows the connections of all coils belonging to phase A for a 12-pole configuration. Superscript (′) indicates the beginning of a coil; (″) indicates the coil end. In the above schematic, coils #1, #7, #13, #19, #25 and #31 are positively wound coils while coils #4, #10, #16, #22, #28 and #34 are negatively wound coils. Note that one end of each coil is connected to the mid-point of each inverter branch, while the other is tied to the motor neutral. The inverter has a total of 72 switches.
With the high-speed configuration, the machine operates with 9 coils per pole. In this specific example, there are 9 phases, each having four coils (two wound in a positive and two in a negative direction). Coil placement and direction in which the coils are wound are given in FIG. 3. As before, the (+) or (−) sign associated with each phase, specifies whether the coils is wound in a positive or negative direction. For example, phase A consists of positively wound coils 1 and 19 and negatively wound coils 10 and 28. By comparing the coils having the same number (FIGS. 1 and 3) one can see that the direction in which each coil is wound does not change when machine number of poles is changed, meaning that pole changing is achieved only by appropriately connecting the fixed wound coils. (For example, coil #5 is always wound in a positive direction; coil #2 is always wound in a negative direction, etc.)
FIG. 4 shows connection of all coils belonging to phase A for this example of 4-pole connection. The change in pole numbers is achieved through inverter control—by selecting the sequence in which the coils are energized. The coils are re-arranged and re-assigned to appropriate phase through inverter control.
The above example, with the number of poles, number of phases and number of stator slots, is used only to illustrate the principle of Pole-Phase Modulation method. The pole changing is not restricted to configurations described above—for example, the 4-pole configuration can be also realized with a 3-phase winding. It is the combination of toroidally wound motor and inverter supply which gives the required flexibility of reconfiguring the machine winding by appropriately connecting the selected coils.
There are two basic methods for implementing vector control:                1. The indirect method, by which a specific, pre-calculated slip speed is imposed on the motor. As long as the correct slip speed is maintained (during both transients and in steady state), the drive operates with de-coupled (independent) control of motor output torque and rotor flux.        2. The direct method, by which a position of the rotor flux is either directly measured or is calculated from measurement of other motor variables. If such measurement does not include measurement of motor speed or position, the control is called “sensorless”. While rotor flux is most commonly used, the airgap or stator flux can be also calculated or measured. The flux position is then used for a correct orientation of the drive control.        
However these methods of vector control have never been used in connection with induction machines and Pole-Phase Modulation.
Toroidally wound induction machines with Pole Phase Modulation have been disclosed in U.S. Pat. No. 5,977,679. This disclosure is incorporated herein by reference.